.alpha.-Olefin oligomers have been used as functional fluids and lubricating oils. Many methods for making these oligomers are known. In U.S. Pat. No. 2,937,129 a C.sub.5-14 .alpha.-olefin is oligomerized using a dialkyl peroxide catalyst. U.S. Pat. No. 3,149,178 describes a process of oligomerizing a C.sub.6-12 .alpha.-olefin using a peroxide catalyst, Friedel-Crafts catalyst or heat followed by removal of dimer and hydrogenation of the residual product. U.S. Pat. No. 3,179,711 describes a process using as the catalyst the reaction product of TiCl.sub.4, alkyl aluminum sesquihalide and tetraalkyl silicate. U.S. Pat. No. 3,330,883 describes a process for making an .alpha.-olefin oligomer without using expensive 1-dodecene by oligomerizing a mixture of at least one C.sub.6-10 .alpha.-olefin and at least one C.sub.14-16 .alpha.-olefin using a Friedel-Crafts catalyst. U.S. Pat. No. 3,382,291 describes an oligomer process in which a stream of boron trifluoride promoter complex are concurrently added to a reaction zone. In U.S. Pat. No. 3,576,898 a C.sub.24-60 oligomer is made by dimerizing an .alpha.-olefin using a Ziegler catalyst followed by dimerization or trimerization of the dimer using a Friedel-Crafts catalyst.
The following patents are but a few of the many other methods described for making oligomers: U.S. Pat. Nos. 3,682,823; 3,763,244; 3,769,363; 3,780,128; 3,798,284; 3,884,988; 3,097,924; 3,997,621; 4,045,507 and 4,045,508.
In many uses it is preferred that the oligomer have a low viscosity, for example, below about 5 cs, and most preferably about 4 cs, at 210.degree. F. These low viscosity oligomers are especially useful as engine lubricating oil in that they minimize friction and thus improve fuel economy. Used either alone or as blends with mineral oil they can provide lubricating oils with viscosities which qualify as SAE 5W 30 crankcase oils.
In attempting to make such low viscosity oligomers, an ever present problem is the volatility of the oligomer. As the oligomers are made less viscous their volatility increases such that a portion of the oligomer will vaporize at engine crankcase temperature. This results in apparent oil consumption and if excessive, can result in failing to pass the required ASTM IIId engine test.
If the volatile components are distilled out prior to blending the oil in order to avoid excessive vaporization, the viscosity of the oligomer increases sharply making them unsuitable for blending low viscosity oils. Thus, the oligomer producer finds himself on the horns of a dilemma.
In the past, useful oligomers having the desired viscosity and volatility properties have been made by oligomerizing 1-decene using a Friedel-Crafts catalyst followed by distillation of dimer and hydrogenation of the residue. However, 1-decene is in limited supply because it is a coproduct made together with a broad range of .alpha.-olefins. In order to provide more flexibility in making synthetic lubricating oil there exists a need for a process capable of using a broad range of .alpha.-olefins to produce an oligomer which will have the desired low viscosity, low volatility physical properties needed for a premium lubricating oil. The present invention provides such a process.